1. Field of the Invention
This invention relates to a process for obtaining streams of alcohols of different characteristics (methanol, ethanol, n-propanol, isobutanol) from mixtures thereof with water and other high-boiling or low-boiling organic compounds.
2. Discussion of the Background
Mixtures of methanol, ethanol, propanol, isobutanol and other compounds can be produced with greater or lesser quantities of water (by operating within suitable T and P ranges in the presence suitable catalysts) from synthesis gas obtained by partial oxidation of methane or naphtha, by methane steam reforming or by coal gasification.
Such mixtures can be used to obtain high-octane synthesis products and in particular methyl and ethyl ethers (MTBE and ETBE), these being products of ever increasing interest in the light of recent vehicle fuel legislation, both because of their octane characteristics and because of their oxygen content.
However the demand for MTBE in particular is increasingly difficult to cover by synthesis via the addition of methanol to the isobutene present in the C.sub.4 streams from S.C. (steam cracking) and F.C.C. (fluid catalytic cracking) due to the poor availability of isobutene.
Alcohol mixtures obtained from CO and H.sub.2 can also be used as sources of isobutene if the isobutanol contained in them is separated and dehydrated to olefin. In this manner such alcohol mixtures become a source of both the raw materials required for producing MTBE, ETBE or their mixtures, and hence become a strategic alternative to the classical production of these compounds by S.C. and F.C.C.
To make the synthesis of high-octane products such as MTBE and ETBE from alcohol mixtures obtained from CO and H.sub.2 economically interesting, the available streams must be such as to allow certain restrictions to be satisfied, namely:
methanol can be used either alone or in mixture with ethanol, however in either case the water level must be very low; likewise in both cases the C.sub.3 alcohol level must be minimized as it reacts with iso-olefins with unfavourable thermodynamics. PA1 the propanol is preferably recycled to the reactor in which the alcohols are synthesized from CO and H.sub.2 as this results in an increase in isobutanol production. If recycled to the alcohol synthesis reactor this stream can contain even considerable quantities of methanol and ethanol, the former being recovered as CO and H.sub.2 and the latter being converted into isobutanol and hence into a higher value product. PA1 the isobutanol (and any other high-boiling products present) fed to dehydration to obtain isobutene must have negligible C.sub.2 and C.sub.3 light alcohol content to prevent production of light olefins which cannot be etherified by the catalyst system usually used for etherification to MTBE and ETBE and hence of no interest. However this stream can accept the presence of high-boiling products (oxygenated or not).
The ethanol distribution between the stream withdrawn from the side take-off of the 1st column and the overhead stream from the 2nd column depends essentially on whether ethanol is to be produced in order to obtain ETBE in mixture with MTBE or whether isobutanol is to be produced in order to obtain more isobutene and hence more MTBE.
The separation cycle of the present invention provides a process which satisfies all the aforesaid restrictions. Separation from a mixture of propanol, water, isobutanol and other heavy compounds is difficult and costly, particularly because of the formation of homogeneous and heterogeneous binary azeotropes with the water.
In this respect, the difference in boiling point between the azeotropes of nC.sub.3 OH and isobutanol with water is only about 3.degree. C. (Azeotropic Data Vol. III No. 35, Advances in Chemistry series), the isobutanol azeotrope being heterogeneous. The difficulties of separating the two are further aggravated by hydraulic and mechanical problems inherent in columns with phase separation (the behaviour of any high-boiling compound towards water is very similar to that of isobutanol).
The current art would suggest separating the methanol as overhead in a first column, then the water by azeotropic distillation in the presence of a suitable entrainer additive (using two columns), and then the ethanol and propanol in a further two fractionating columns (the isobutanol remains as bottoms in the last column). This cycle is of extremely high capital and operating cost, due mainly to the water separation.